Continuous separation processes for the selective adsorption of para-xylene from a mixture of other xylene isomers, ethylbenzene, and non-aromatic hydrocarbons are common in industry. Generally, the processes use a solid adsorbent which preferentially retains the para-xylene in order to separate the para-xylene from the rest of the mixture. Often, the solid adsorbent is in the form of a simulated moving bed, where the bed of solid adsorbent is held stationary, and the locations at which the various streams enter and leave the bed are periodically moved. The adsorbent bed itself is usually a succession of fixed sub-beds. The shift in the locations of liquid input and output in the direction of the fluid flow through the bed simulates the movement of the solid adsorbent in the opposite direction. Moving the locations of liquid input and output is accomplished by a fluid directing device known generally as a rotary valve which works in conjunction with distributors located between the adsorbent sub-beds. The rotary valve accomplishes moving the input and output locations through first directing the liquid introduction or withdrawal lines to specific distributors located between the adsorbent sub-beds. After a specified time period, called the step time, the rotary valve advances one index and redirects the liquid inputs and outputs to the distributors immediately adjacent and downstream of the previously used distributors. Each advancement of the rotary valve to a new valve position is generally called a valve step, and the completion of all the valve steps is called a valve cycle. The step time is uniform for each valve step in a valve cycle, and is generally from about 60 to about 120 seconds, such as 90 seconds. A typical process contains 24 adsorbent sub-beds, 24 distributors located between the 24 adsorbent sub-beds, two liquid input lines, two liquid output lines, and associated flush lines.
The principal liquid inputs and outputs of the adsorbent system consist of four streams: the feed, the extract, the raffinate, and the desorbent. Each stream flows into or out of the adsorbent system at a particular flow rate, and each flow rate is independently controlled. The feed, which is introduced to the adsorbent system, contains the para-xylene (PX) which is to be separated from other components in the feed stream, which typically include ethylbenzene (EB), metaxylene (MX), orthoxylene (OX), toluene, various C9+ aromatics, and non-aromatics. The desorbent, which is introduced to the adsorbent system, contains a liquid capable of displacing feed components from the adsorbent. The extract, which is withdrawn from the adsorbent system, contains the separated para-xylene which was selectively adsorbed by the adsorbent, and desorbent liquid. The raffinate, which is withdrawn from the adsorbent system, contains the other xylene isomers, ethylbenzene, non-aromatic hydrocarbons which were less selectively absorbed by the adsorbent, and desorbent liquid. There also may be associated flush streams introduced to and withdrawn from the adsorbent system. These flush streams can vary in composition and rate and can include but are not limited to paraxylene, ethylbenzene, metaxylene, orthoxylene, and desorbent. The flush flow rates are typically independently controlled. The four principal streams are spaced strategically throughout the adsorbent system and divide the sub-beds into four major zones, each of which performs a different function.
Zone I contains the adsorbent sub-beds located between the feed input and the raffinate output, and selective adsorption of the para-xylene takes place in this zone. Zone II contains the adsorbent sub-beds located between the extract output and the feed input, and the desorption of components other than para-xylene takes place in this zone. Zone III contains the adsorbent sub-beds located between the desorbent input and the extract output, and the para-xylene is desorbed in this zone. Finally, Zone IV contains the adsorbent sub-beds located between the raffinate output and the desorbent input, and the purpose of this zone is to prevent the contamination of the para-xylene with other components. The flush flows are introduced in the sub-beds of some of the major zones and create minor zones which are a function of the major zone rates and the smaller flush flow rates.
Two other important streams are the pumparound and pusharound streams. In a typical para-xylene separation process the adsorbent bed consisting of 24 sub-beds is split into two main chambers. One chamber contains sub-beds 1 through 12 and the other contains sub-beds 13 through 24. Although functionally the adsorbent system as a whole does not have a top or a bottom, each chamber has a physical top and bottom. The pumparound and pusharound streams each conduct the liquid effluent exiting the physical bottom of one adsorbent bed chamber back up to reenter the physical top of the other adsorbent bed chamber. The pumparound stream is the stream that conducts the effluent of sub-bed 24 from the physical bottom of the second chamber to reenter sub-bed 1 at the physical top of the first chamber. The pusharound stream conducts the effluent of sub-bed 12 from the physical bottom of the first chamber to reenter sub-bed 13 at the physical top of the second chamber. It is important to note that the composition of the pumparound or pusharound stream changes with each valve step, and in one valve cycle both streams will have sequentially carried the composition which corresponds to each valve position.
In this regard, see U.S. Pat. No. 5,470,480 and references cited therein.
The common practice in industry is to control the para-xylene simulated moving bed separation process either by on-line gas chromatography analyses, by off-line laboratory analyses of the product streams or by online gas chromatography of the product streams. When controlling on-line, the gas chromatography analysis of the pumparound stream generally requires about 10 minutes which is considerably greater than the usual step time of the rotary valve. Therefore, only select valve positions may be sampled and analyzed. Generally, only Zone II and Zone IV are sampled and analyzed. The data provided by this on-line gas chromatography procedure is useful for detecting process upsets, but unfortunately analyzing the composition of only two valve positions provides limited information regarding the performance of the separation process.
A more thorough control is accomplished using off-line laboratory gas chromatography analyses to determine the values of the concentrations of the components in samples of the pumparound stream taken at each valve position in a valve cycle. The measured concentrations are then plotted versus their relative valve positions to form what is generally called a profile. Using the profile, the recovery and purity of the para-xylene can be calculated and the degree of optimization of the separation may be visually assessed. Then required changes in the step time and liquid stream flow rates may be determined and implemented. The drawbacks to controlling a separation process in this fashion are the time delay between sampling and analytical results where the latter are used to determine whether or what changes should be made, the labor involved to manually collect the stream samples, and the personal exposure of the operator manually collecting the stream samples. Since the analyses are performed off-line, the time delay may be from one to several days long. Because of the drawbacks, refiners generally only perform this procedure infrequently to determine the health of the separation process, such as about once every six months, or if there is a problem with the separation process.
Other separation processes, such as the separation of oil from wax, have been controlled using spectroscopic determinations of impurities in the separated pure product. For instance, the Canadian Patent Application 2,050,108 disclosed spectroscopically measuring one component of a mixture in another component of the mixture following the separation of the mixture into its components. The results of the measurements are used to control the separation so that the amount of impurity in the pure product is controlled to a desired value.
A paraxylene process such as described above can be controlled in an open loop fashion where the operator adjusts process parameters based on the product purity and product recovery analyzers. There is a time lag between parameter adjustments and final product purity and recovery due to the process. Due to normal variations in feed rate, feed composition, and other variables, the operator leaves a cushion between the target purity and recovery and the actual product purity and product recovery. This cushion requires more energy and can limit production.
Additional relevant patents are U.S. Pat. Nos. 5,470,482; 5,457,260; 6,072,576; 6,162,644; 6,217,774; 7,192,526; and U.S. Patent Application Publication Nos. 20060006113 and 20070119783.
The present inventors have recently described the use of analyzers in the Parex process using predictive models in U.S. Provisional Application No. 61/182,466, filed 29 May 2009. As described therein, a process control application can be constructed that automatically adjusts the process parameters to meet the product purity and recovery targets. The application also uses the analyzers to adjust the parameters to correct any prediction errors. Because the controller can decrease variability in the product purity and recovery, the targets can be run closer to the purity specification and recovery needs. This saves energy and can increase production.
The present inventors have now discovered an improved feedforward algorithm including an adjustment to one of the controlled variable values for feed composition with improved corrective controls that automatically adjusts parameters to decrease variability in product purity.